Medical diagnostic imaging has evolved as an important non-invasive tool for the evaluation of pathological and physiological processes. Presently, nuclear magnetic resonance imaging (MRI) and computerized tomography (CT) are two of the most widely used imaging modalities. Although both MRI and CT can be performed without the administration of contrast agents, the ability of many contrast agents to enhance the visualization of internal tissues and organs has resulted in their widespread use.
Proton MRI is based on the principle that the concentration and relaxation characteristics of protons in tissues and organs can influence the intensity of a magnetic resonance image. Contrast agents which are useful for proton MRI effect a change in the relaxation characteristics of protons which can result in image enhancement and improved soft-tissue differentiation. Different classes of proton MR imaging agents include paramagnetic metal chelates and nitroxyl spin labelled compounds.
Two commercially available paramagnetic chelates are PROHANCE (Squibb Diagnostics, Princeton, N.J.) and MAGNEVIST (Berlex, Wayne, N.J.). (See also, inter alia, H. J. Weineman et al., Am. J. Roentgenol. 142:619-624, 1984; M. M. Le Mignon, et al., Investigative Radioloqy 25:933, 1990; A. D. Sherry et al., U.S. Pat. No. No. 5,316,757, issued 1994; and A. D. Sherry et al., PCT Application Ser. No. WO 92/08725, published 1992.)
Examples of nitroxyl spin labeled compounds are described by R. C. Brasch et al., Radiologoy 147:773-779, 1983; G. M. Rosen, U.S. Pat. No. 4,834,964, issued 1989; G. M. Rosen et al., U.S. Pat. No. 5,104,641, issued 1992; J. F. W. Keana et al., U.S. Pat. No. 4,863,717 issued 1989; G. M. Rosen, U.S. Pat. No. 5,256,397 issued 1993; Y. Berchadsky et al., U.S. Pat. No. 5,006,663, issued 1991; and I. B. Leunback, PCT Application Ser. No. WO 90/00904, published 1990.
Fluorine (.sup.19 F) MRI is also in the early stages of development. Because of the 100% natural abundance of .sup.19 F and the complete absence of biological background, .sup.19 F MRI promises to be an important diagnostic imaging tool of the future. Fluorine-containing imaging agents include perfluoro-tert-butyl containing organic compounds (W. J. Rogers, Jr., et al., U.S. Pat. Nos 5,116,599 issued 1992, 5,234,680 issued 1993, and 5,324,504 issued 1994) and fluoro-substituted benzene derivatives (P. Blaszkiewicz et al., U.S. Pat. No. 5,130,119 issued 1992.)
CT is based on the principle that various substances effect different degrees of attenuation of an X-ray beam. Contrast agents useful for CT usually contain atoms which are electron dense, such as bromine or iodine, and are efficient attenuators of X-ray radiation. By far the most common CT agents are monomeric or dimeric iodinated benzene rings with various pendent groups such as ORAGRAFIN, CHOLOGRAFIN and RENOGRAFIN (Squibb Diagnostics, Princeton, N.J.). One important advance in the use of iodine-containing CT agents has been the development of non-ionic contrast agents, such as the ones described by M. T. Kneller et al., PCT Application Ser. No. WO 93/10825 published 1993.
The usefulness and efficiency of chemical compounds as contrast agents depends on their ability to exhibit a predictable and desirable biodistribution and metabolism in vivo. Their behavior in vivo depends on parameters such as molecular weight, charge, osmolality, hydrophobicity, partition coefficient, susceptibility to metabolic breakdown, and tissue or organ targeting efficiency. In order to improve their solubility and biodistribution, many contrast agents are used in conjunction with delivery systems such as emulsions, liposomes, and microparticles. Others are combined with polymeric systems which allow complex contrast agents to be designed with specific molecular weight, charge and targeting characteristics. For example, contrast agents can be conjugated to dense star polymers (see, for example, Tomalia et al., U.S. Pat. No. 5,338,532, issued 1994) or amino acid polymers (D. Meyer, et al., PCT WO 93/10824, published 1993).
The present invention relates to novel CT and MRI agents which are in the form of calixarene conjugates. Calixarenes are macrocycles comprising phenolic units ortho-linked by methylene bridges, as represented by the following formula: ##STR2## wherein n is typically 4, 5, 6, 7, or 8, and more commonly 4, 6, or 8. Calixarenes are commonly referred to as calix[n]arenes wherein n refersto the number of phenolic units. As denoted herein, the phenolic --OH group occupies the 1-position, and the substituent --R group occupies the 4-position. Although different conformations are possible depending on the type and degree of derivatization, calixarenes are often described as being basket- or cup-shaped, with a larger diameter upper rim comprised of substituents at the 4-positions and a smaller diameter lower rim comprised of substituents at the 1-positions.
Calixarenes were first discovered in the 1940's (see, inter alia, J. B. Niederl et al., J. Am. Chem. Soc., 62:2512-2514, 1940). A variety of calixarenes and calixarene derivatives have been prepared and characterized (see, inter alia, C. D. Gutsche, Calixarenes, 1989, Royal Society of Chemistry, Cambridge, UK; and Z. Asfari et al., Jansen 24 Chimica Acta, 10(1):3-10,1992) and include, for example, alternative substituents at the 1-and/or 4-positions, and alternative ortho-linkages, such as --(C.dbd.O)--, --CH.sub.2 CH.sub.2 --, and --CH(CH.sub.3)--.
Calixarenes have found use in catalysis (polymerization accelerators), transport and extraction of metallic cations (cesium ion extraction, metal ion sequestrants), and in modifying the chemical properties of polymers, drugs, and dyes (see, inter alia, Z. Asfari etal., supra; and W. I. Hwang etal., PCT Application Ser. No. WO 94/03164 published 1994).
Recently, Bakker etal. (J. Org. Chem., 59:972-976, 1994) have disclosed the synthesis of radionuclidic "catixspherands", which are capable of forming stable complexes with radionuclides such as .sup.81 Rb.sup.+. These calixspherands are composed of a calixarene backbone which is conjugated to a m-terphenyl moiety. The m-terphenyl moiety is subsequently derivatized and linked to a low molecular weight protein (LMWP) which facilitates organ (in this case, kidney) targeting. The calixspherand-LMWP conjugate thus formed is then complexed with .sup.81 Rb.sup.+ and used in conjunction with a scintillation detection for the determination of blood flow in tissue and organs.
Heretofore, the use of calixarene conjugates as MRI or CT imaging agents has not been reported.
Disclosure of the Invention
The present invention relates to calixarene conjugates useful for imaging, particularly magnetic resonance imaging (MRI) and computed tomography (CT).
Accordingly, one aspect of the invention relates to calixarene conjugates comprising: (i) a calixarene backbone; and (ii) at least one imaging moiety linked thereto. Preferably, at least one imagining moiety is an MR imaging moiety or a CT imaging moiety.
Another aspect of the invention relates to calixarene conjugates of the formula: ##STR3## wherein at least one of the R.sup.1 and R.sup.4 substituents comprises an imaging moiety, the remaining R.sup.1 and R.sup.4 substituents, if any, are spectator groups, J is an ortho-linker, and n is an integer from 4 to 8.
Yet another aspect of the invention relates to calixarene conjugates useful for CT imaging wherein the imaging moiety comprises two or more iodine atoms.
Still another aspect of the invention relates to calixarene conjugates useful for MRI wherein the imaging moiety comprises at least one of (i) an organic moiety comprising four or more fluorine atoms; (ii) a nitroxyl spin labeled moiety; or (iii) a metal chelate moiety.
Yet another aspect of the invention relates to imaging agent formulations comprising a calixarene conjugate comprising a calixarene backbone and at least one CT or MR imaging moiety linked thereto, and a pharmaceutically acceptable carrier.
Still another aspect of the invention relates to methods of CT and/or MR imaging comprising the steps of (i) administering an effective amount of a calixarene conjugate of the invention; and (ii) acquiring a CT and/or MR image of the subject while the calixarene conjugate is present in the body.